1. Field of the invention
The present invention relates to a zoom optical system used for an optical apparatus such as an image projecting apparatus, an exposure apparatus and the like.
2. Related background art
Some optical apparatuses as described above need a zoom optical system that is excellent in the telecentricity on the object side and excellent, in invariance of positions of the object surface, the image plane and the exit pupil to a variation of the focal length.
For example, Japanese Patent Application Laid-Open No. 2002-207167 has disclosed a zoom optical system suitable for illumination optical system installed in a projection exposure apparatus, that moves a lens unit having negative refractive power or a lens unit including a lens element having a strong negative refractive power toward an object side as focal length gets shorter so as to make positions of an object surface, an image plane, an entrance pupil and an exit pupil immobile against variation of focal length.
In addition, in Japanese Patent Application Laid-Open No. 2002-055279, zoom optical system suitable for a transmission optical system in illumination optical system installed in a projection exposure apparatus has been disclosed. The zoom optical system includes at least four lens units and at least three lens units moves so as to change the refractive power arrangement from negative-positive-positive or positive-positive-negative in the order from the object side to a positive-negative-positive in the order from the object side during zooming from the wide angle end to the telephoto end and thereby positions of an object surface, an image plane, an entrance pupil and an exit pupil are made immobile against variation of focal length.
Here, it is considered to use a zoom optical system disclosed in Japanese Patent Application Laid-Open No. 2002-207167 and Japanese Patent Application Laid-Open No. 2002-055279 in projection optical system or a part thereof of an image projecting apparatus of magnifying/projecting an image of an original formed on a liquid crystal panel.
However, if liquid crystal panel is disposed on a reduction side focal plane of a zoom optical system disclosed in Japanese Patent Application Laid-Open No. 2002-207167, it is impossible to secure a, sufficient back focus and space for arranging a color synthesizing member at the reduction side. For a so-called 3-plate type image projecting apparatus, a color synthesizing member for synthesizing three color lights of red, green and blue is arranged on the reduction side of a zoom optical system. However, if there is no sufficient back focus as described above, it will become impossible to secure a space for arranging the color synthesizing member.
In addition, in the zoom optical system disclosed in Japanese Patent Application Laid-Open No. 2002-207167, a lens unit at the most reduction side has a negative refractive power, and as focal length gets shorter, a lens unit having a positive refractive power arranged on a magnification side of the negative lens unit (a second lens unit) moved toward the magnification side so that an interval between the both lens units increases. Therefore, the effective diameter of the positive lens unit will get large. Moreover, since the lens unit at the most reduction side has a negative refractive power, a numerical aperture (NA) on the reduction side will get small.
Here, in Japanese Patent Application Laid-Open No. 2002-207167, such a case where a lens unit at the most reduction side is caused to have a positive refractive power is disclosed. However, since this lens unit significantly moves toward the magnification side as the focal length gets shorter, the effective diameter of the lens unit will get large if the NA on the reduction side is made large.
That is, in this zoom optical system, in a state of the minimum focal length (wide), a plurality of lens units get closer toward an aperture stop side, and as the focal length gets longer, move toward the reduction side focal plane side, and therefore the effective diameter of the lens unit is apt to get large, which is not appropriate for deriving compactness. Moreover, that is not appropriate either for securing a sufficient back focus since a lens unit on the most magnification side has a positive refractive power. Furthermore, the zoom optical system assumes a light source with single wavelength, and is inappropriate for a projection display of a color image.
In addition, in the zoom optical system disclosed in Japanese Patent Application Laid-Open No. 2002-055279, a synthesized refractive power of the adjacent lens units get occasionally, negative, that is, the refractive power is weak. Accordingly, the focal length is large as long as 190 mm at minimum. This takes place since the zoom optical system is an optical system intended to be appropriate for an exposure apparatus, and this system is not suitable for an optical system such as a projector etc. that is desired to derive compactness and a wide angle. In addition, likewise the zoom optical system disclosed in Japanese Patent Application Laid-Open No. 2002-207167, this zoom optical system assumes a light source with a single wavelength, and therefore is inappropriate for a projecting display of a color image.